Immune mechanisms of Pathogenesis and Protection for Crohn's Disease Candidate Microbial Antigens. Commensal bacteria are an important disease factor in human Crohn's disease and murine IBD models. This insight raises two critical issues for emerging research: Is immune pathogenesis of CD mediated by responses to particular enteric microorganisms? And, Can these responses by manipulated to yield now- inflammatory or anti-inflammatory immunity. In the previous grant period, our laboratory successfully applied immunologic and subtractive cloning approaches identify a set of bacterial species and microbial antigens associated with UC and CD. This renewal application focuses on one of these candidates, I2, a novel gene of bacterial origin found to be localized in CD lesions. Recombinant expression of the I2-encoded protein revealed highly disease-specific antibody levels in human CD. In the mouse, native memory T cell immunity to I2 was demonstrated, and the effector profiles were polarized to TH1 and TH2 in colitis susceptible and resistant mouse strains, respectively. These finding support the bacterium as a candidate pathogen in CD, and present the unique opportunity to evaluate the microbiology and immune response for a candidate human CD pathogen in laboratory mouse strains. Our renewal project tests two hypotheses. First, we predict that the I2 bacterium exemplifies one of the subset of microorganisms pathogenic in Crohn's disease, due to its capacity to colonize relevant mucosal sites and elicit a local tissue-destructive mucosal sites and elicit a local tissue-destructive immune response in susceptible hosts. We will experimentally approach this hypothesis by isolating and characterizing the I2 bacterium with regard to taxonomy, virulence traits, and intestinal tissue distribution. From an immunologic perspective, we will evaluate the peptide specificity, ontogeny, anatomic origin, and pathogenicity of T cell populations and cell lines specific for the I2 antigen. Second, we predict that this or other antigen commensal microorganisms and mucosal autoantigens are targets of anti-inflammatory T cells necessary for the natural and therapeutic protection to colitis. We will experimentally test this hypothesis by characterizing the ontogeny and localization of these T cell populations; isolating antigen-specific anti-inflammatory T cell populations and evaluating their protective effect in colitis transfer models; and, testing antigen-transfer strategies as therapeutic immunomodulators of model colitis. These aims will depend on shared studies with Project 4 (cellular and topologic development of pathogenic and protective mucosal T cell populations in the mouse), Project 2 ( characterization of human B and T cell clonal populations specific for Project 3-derived microbial antigens), and Project 1( human genetic loci associated with these anti-microbial responses).